Technical Field
The present disclosure relates to an electromagnetic flow meter, and more particularly, to a technology of suppressing deformation of a lining material configured to cover an inside of a measurement tube.
Related Art
Since an electromagnetic flow meter configured to measure a flow rate of conductive fluid by using electromagnetic induction is robust and has high precision, it is widely used for an industrial use. The electromagnetic flow meter is configured to enable the conductive fluid to be measured to flow into a measurement tube, to which a magnetic field is applied in an orthogonal direction, and to measure a generated electromotive force. Since the electromotive force is proportional to a flow rate of the fluid to be measured, it is possible to obtain a volumetric flow rate of the fluid to be measured on the basis of the measured electromotive force.
In the electromagnetic flow meter, the measurement tube having an electrode for electromotive force measurement and the like attached thereto is coupled to a piping mounted to a plant and the like, and is classified into a flange type and a wafer type, depending on a coupling structure thereof. In the flange type, the measurement tube of the electromagnetic flow meter is formed with a large flange and a bolt is enabled to pass through a flange of the piping and the flange of the measurement tube, so that the measurement tube is coupled to the flange of the piping. In the wafer type, the measurement tube of the electromagnetic flow meter is formed with a small flange and is coupled to a flange of the piping without enabling a bolt to pass through the flange of the measurement tube.
FIG. 13 illustrates a coupling structure of a flange-type electromagnetic flow meter to a piping. As shown in FIG. 13, an electromagnetic flow meter 200 has a converter 210 and a measurement tube 220, and both ends of the measurement tube 220 are formed with flange parts 221. The respective flange parts 221 are coupled to piping-side flange parts 311 by bolts 321 and nuts (not shown).
When coupling the electromagnetic flow meter-side flange part 221 and the piping-side flange part 311, they are tightened by the bolt 321 and the nut with an earth ring 222 and a gasket 312 being sandwiched therebetween. The gasket 312 is used to secure liquid tightness of the coupled part. The earth ring 222 is a circular metal plate (ring plate) having a round hole perforated therethrough and corresponding to a size of the piping and is used to equalize an earth of a fluid to be measured and a common potential.
As shown in FIG. 14, in a wafer-type electromagnetic flow meter, a flange part 241 of a measurement tube 240 has a smaller diameter and a bolt 342 is not enabled to pass through the flange part 241 of the measurement tube 240. For this reason, the measurement tube 240 is fixed by being sandwiched between both piping-side flange parts 341 which are fastened by bolts. At this time, a centering device 344 for positioning may be used.
Also in the wafer-type electromagnetic flow meter, when coupling the electromagnetic flow meter-side flange parts 241 and the piping-side flange parts 341, they are tightened by bolts 342 and nuts with earth rings 242 and gaskets 343 being sandwiched between the electromagnetic flow meter-side flange parts 241 and the piping-side flange parts 341.
FIG. 15 is a sectional view of the coupled part between the electromagnetic flow meter-side flange part 241 and the piping-side flange part 311. Here, the flange-type electromagnetic flow meter is exemplified. As shown in FIG. 15, a lining material 223 is bonded or welded to an inside of the measurement tube 220 of the electromagnetic flow meter 200, including a part of an area of a coupling side of the flange part 221. The lining material 223 is used not only to secure insulation of the measurement tube 220 to the electromotive force but also to secure corrosion resistance and wear resistance of the measurement tube 220 against the fluid to be measured. The lining material 223 is selected depending on the fluid to be measured. For example, fluorine resin (PFA, PTFE), polyurethane rubber, soft natural rubber and the like may be used.
The lining material 223 is formed to cover up to an inner periphery area having a predetermined diameter of the coupling side of the flange pan 221. Therefore, when the earth ring 222 is attached from above the lining material 223 and the electromagnetic flow meter-side flange part 221 and the piping-side flange part 311 are coupled to each other, the earth ring 222 and the lining material 223 are closely contacted.
[Patent Document 1] Japanese Patent Application Publication No. 2010-151648A
In a case where the lining material 223 having high ductility such as rubber is used, when a pressure of the fluid to be measured is high, the lining material 223 is deformed to expand in an outer periphery direction due to an inner pressure, as shown in FIG. 16. When the lining material 223 is bulged in the outer periphery direction, a thickness of the lining material 223 is reduced as much as that. Therefore, a surface pressure between the earth ring 222 and the lining material 223 is lowered, so that the liquid tightness may be influenced.
From a standpoint of safety, even if the pressure of the fluid to be measured higher than a limit value on the specification is applied, the deformation of the lining material 223 is preferably suppressed so as to keep the liquid tightness.